5. CHEMICAL PROPERTIES OF SOIL

5.1 Colloidal properties:

A major portion of the clay fraction (2mu) exists in the colloidal state (1 to 200 mu). This fraction exhibit the typical polloidal properties such as Tyndall effect, Brownian movement and possession of electric charges.

5.2 Ion adsorption:

The soil particles due to the presence of charges adsorb and exchange ions in solution. When fertilizers are added to agricultural soils or ponds, most of the nutrients in the fertilizer (cations as well as anions) are adsorbed on the negatively and positively charged sites of the soil or pond mud and released slowly to the soil water or pond water over a long period of time. This explains why fertilizers added to a pond may remain active for many years. In the case of phosphorus fertilizer, not much of the original fertilizer applied is washed out of the pond at each draining. The pond mud acts as a buffer system for many elements which could control the concentrations in the overlying waters because of the large concentrations of some elements present in the mud sediments. The effects of this buffer system could be to keep the concentrations in the overlying waters relatively constant even though the concentrations of the element in the water is altered. The large quantity of lime required to increase the pH of ponds is often due to the neutratization of the potential or exchange acidity resulting from adsorbed hydrogen and aluminium ions.

5.3 Origin of charges in soil and ion exchange:

Electric charges on soil colloids arise from principally three sources:

1. from isomorphous substitution of one ion by another of different valency within the clay mineral structure. This gives rise to mainly negative charges. The charges are permanent and do not change with change in pH of the external solution.

2. from ionisation of OH groups attached to the Al, Si, Fe at the edges of clay minerals. The charges created by this process are negative, zero or positive depending on the pH. Ionisation of OH groups attached to the Al at the edge of clay minerals can be represented as follows:

   Clay - Al - OH+2	Clay - Al - OH	Clay - Al O-
   Very low pH	Intermediate pH	High pH
   (below 3)	(about 3)	(above 3)

   The pH at which the soil has net zero charge, positive charge or negative charge depends on the type and amount of the various clay minerals present in the soil.

3. from ionisation of - NH₂, - OH and - COOH functional groups in the organic matter of soils. This too gives rise to positive, neutral and negative charges as in (ii) depending on the pH.

   Organic matter - COOH+2	Organic matter - COOH	Organic matter - COO-
   (pH < 2)	(pH < 2)	(pH < 2)

   In a soil, charges arise from all the above 3 sources. Negative charges increase with increase in pH of the water surrounding the soil particles and positive charges increase with decrease in pH (Fig. 1.11). At very low pH, the soil or pond mud adsorbs anions and acts as an anioh exchanger (Fig. 1.12). At higher pH, it adsorbs cations and acts as a cation exchanger (Fig. 1.13).

As the pH of most agricultural soils and ponds muds are generally higher than 4, the soil or mud is principally a cation exchanger, though it has some anion exchange properties. On the negatively charged sites or cation exchange sites, exchangeable cations are adsorbed and on the positively charged sites or anion exchange sites, exchangeable anions are adsorbed. Exchangeable cations can be acidic or basic cations. Acidic cations are those which produce acidity in soil. Common exchangeable cations and anions in soils are given below:

Exchangeable cations:

Exchangeable anions: SO^2-₄, BO^2-₃, CO^2-₃, HCO₃^-, OH^-, H₂PO₄^-, HPO^2-₄, PO^3-₄, etc.

The quantity of cations which are adsorbed on the muds is expressed as milliequivalents of cations per 100 g (meq/100 g) of dry mud and is termed the cation exchange capacity (CEC). CEC is a measure of the total negative charges in the soil. CEC increase with increase in pH, % clay and % organic matter content in the soil.

The fraction of CEC occupied by basic cations is called base saturation and the fraction of CEC occupied by acidic cations is base unsaturation.

A sample calculation of CEC, base saturation and base unsaturation of a soil from Port Harcourt, Nigeria is given in Table IV.

Table IV.: CEC, base saturation and base unsaturation of a Typic Paleudult soil from Port Harcourt, Nigeria

Relationships between base unsaturation and soil pH and between base unsaturation and total hardness of water are presented in Fig. 1.14.

Relative affinities of ions for adsorption on soils depend on the:

1. valency of the ions (higher the valency, higher the adsorptive power)

2. concentration of the ions in the water (higher the concentration, higher the adsorptive power).

3. nature of the ions (i.e. ions having same concentration and valency but different hydrated ionic radii have different affinities for adsorption)

   e.g. Li⁺ < Na⁺ < K⁺ < Rb⁺ < Cs⁺ ;

   Ca²⁺ < Sr²⁺ < Ba²⁺

   Lyotropic series (order of adsorptive power or replacing power). Ions having lower hydrated ionic radii have higher adsorptive power.

Most ions are adsorbed at charged sites on the soil by electrostatic attraction (physical adsorption). Some ions are adsorbed by the formation of chemical bonds at neutral sites in the soil (chemical adsorption). The latter type of adsorption gives rise to fixation of the ion in the pond mud thereby the ion may not become available to the phytoplankton in the water for a long time. Here, the mud acts as a temporary sink for the nutrient (Fig. 1.15).

Fig. 1.11. Influence of pH on surface charge of soil.

Fig. 1.12. Anion exchange equilibria.

Fig. 1.13. Cation exchange equilibria.

Fig. 1.14. Relationships of base unsaturation with pH and total hardness of water

When nutrients in the pond water are removed by phytoplankton or by any other process, the soil exchange complex releases more nutrients to the soil solution phase - the soil exchange complex acts as a store-house for these nutrients. Similarly when lime or fertilizers are added to the pond water, a major portion of these materials get adsorbed on to the pond mud (Fig. 1.16) (Hickling, 1974).

5.4 Soil acidity:

A soil is said to be acidic if its pH is less than 7. Soil acidity can be further divided into:

Acidity in the soil is caused by the following:

1. Adsorbed H⁺, Fe³⁺ and Al³⁺ as well as solution H⁺, Fe³⁺ and

   Al³⁺. Hydrolyses of Fe³⁺ and Al³⁺ produces H⁺

   Al3+	+ H2O	=	Al (OH)2+	+ H+
   Al(OH)2+	+ H2O	=	Al (OH)+2	+ H+
   Al(OH)+2	+ H2O	=	Al (OH)3	+ H+
   			precipitate

2. Leaching of basic cations such as Ca, Mg, K and Na from the soil by heavy rainfall leaving acidic cations to remain in the soil. This is the reason for the occurance of acidic soils in humid regions and alkaline or neutral soils in arid regions.

3. Oxidation of NH⁺₄ and S^2- in soils by micro-organisms.

   NH⁺₄ + 3O₂ = 2NO^-₂ + 4 H⁺ + H₂O

   bacteria involved is nitrosomones

   This reaction along with the following:

   2NO^-₂ + O₂ = 2NO^-₃

   bacteria involved is nitrobacter is called the nitrification process

   Acid sulphate soils on aeration produce acidity by the oxidation of sulphide to sulphuric acid.

   4 Fe S2 + 15O2 + 2H2O = 2 Fe2(SO4)3 + 2 H2SO4
   (pyrite)

   bacteria involved is Thiobacillus ferroxidans

   Fe(SO₄)₃ + Fe S₂ = 3 Fe SO₄ + 2S

   2S + 6 Fe₂(SO₄)₃ + 8 H₂O = 12 Fe SO₄ + 8 H₂SO₄

   Oxidation of sulphur to sulphuric acid is caused by Thiobacillus thicxidans

   S + 3/2 O₂ + H₂O = H₂SO₄

The yellow mottles found in most acid sulphate soils are due to jarosite, a basic ferric sulphate compound K Fe₃(SO₄)₂ (OH)₆ which is produced during pyrite oxidation with the hydrolysis of ferric sulphate and oxidation of ferrous sulphate.

Fe (SO₄)₃ + 2H₂O = 2 Fe (OH) SO₄ + H₂SO₄

4 Fe (SO₄) + 2 H₂O + O₂ = 4 Fe(OH) SO₄.

Soil acidity can be classified into two types:

1. active acidity

2. potential acidity, exchange acidity or reserve acidity.

Active acidity is due to the Al³⁺ and H⁺ (and to a less extent Fe³⁺) in the water surrounding the soil particles and is measured by the pH meter. It is related to the degree of base unsaturation. Potential or exchange acidity is many times higher than the active acidity and is due to the adsorbed H⁺ and Al³⁺. The magnitude of potential acidity is more dependent on CEC than the base unsaturation. Two soils with different CEC may have the same pH and base unsaturation but the soil with greater CEC will have the larger potential acidity.

Fig. 1.15. Phosphate fixation in soil (note adsorption of phosphate ions at neutral sites)

Fig. 1.16. Reaction of lime and fertilizers with soil.

Acidity may give rise to Fe, Al, and Mn, toxicities, phosphorus unavailability (due to fixation), reduction in total hardness and total alkalinity, reduction in nitrification, increase in the amount of undecomposed organic matter, risk of liberation of certain parasitic and bacterial diseases.

5.5 Liming of the soil:

Soil acidity is corrected by the application of lime material. The lime material has to be a calcium or magnesium salt of a weak acid such as limestone (CaCO₃), dolomite (Ca Mg (CO₃)₂), quicklime (CaO), hydrated lime or slaked lime (Ca (OH)₂). The reaction of lime with acidic soil is represented by the following equations

The salts, Ca Cl₂ and Ca SO₄ are not suitable as liming materials as they are salts of strong acids.

In correcting acidity enough lime should be added to neutralize not only the active acidity but also the reserve or potential acidity. Acidity is normally corrected to increase pH to about 5.9 (Boyd, 1979) At this pH, total hardness is about 20 mg/litre and base unsaturation 0.2 (Boyd, 1979). Care should be taken not to overlime the soil as it would create deficiencies of phosphorus and many micronutrients.

Lime requirement of a soil depends on the initial pH of the soil and buffering capacity. Large amounts of lime are required for very low pH soils having high buffering capacity. Buffering capacity of a soil is the resistance of the soil to change its pH on addition of lime or base. Soils having high potential acidity (high CEC) such as clay and high organic matter soils have high buffering capacity (Fig. 1.17).

In the laboratory, lime requirement of a soil is determined by adding certain weight of soil to specified volume of a buffer (para nitrophenol buffer pH 8) and noting the reduction in pH of the buffer.

From the initial pH of the soil and reduction of pH of the buffer solution, the lime requirement could be calculated using standard tables (Adams and Evans, 1962).

Lime is generally applied to the bottom of the dry pond, pond water or water flowing to the pond. As there are several liming materials available for application, the selection of liming material should depend on its purity; rate of reaction with soil and water (fineness); cost; neutralising power (Table V.); handling, storage and bagging considerations; other effects such as control of parasites, gill rot, precipitation of organic matter etc.

Table V.: Neutralising power of lime materials

Note: Neutralising power is inversely proportional to the molecular weight.

5.6 Saline soils:

These are soils having excess soluble salts in the soil, that would impair with normal plant growth. If the salt content is too high, it may affect growth of phyto - and zoo-plankton. High concentrations of some specific elements like boron may also affect the growth of these organisms. The salt content is normally measured by the specific conductivity of soil extract. If the specific conductivity of saturation soil paste extract is more than 4 m mhos/cm, most crops will not grow satisfactory. ppm salt is approximately equal to 640 × specific conductivity.

Fig. 1.17. Buffering capacities of soils with different textures.

Saline soils generally occur in arid and semi-arid regions. It may also occur in localised areas in other regions due to poor drainage etc. These soils can be reclaimed by leaching the salts using good water and removing the leached salts by drains.

5.7 Sodic soils:

These are soils high in pH and exchangeable Na%- pH is generally higher than 8.5 and exchangeable Na occupies more than 15% of CEC. The poor drainage of these soils is not suitable for many upland crops but advantageous for aquaculture. The high Na content may affect the growth of phytoplankton, zooplankton and fish. These soils generally occur in arid and semi arid regions. They are reclaimed by treating the soil with gypsum (Ca SO₄) or sulphur.